Objectives Straw return is an effective practice for improving soil fertility, but the return methods markedly influence rice panicle traits, grain yield, and nitrogen use efficiency. We investigated the suitable methods of straw return for rice in the northern cold region, thereby providing technical support for the rational utilization of straw resources and for increasing rice yield and improving production efficiency.

Methods Field experiments were carried out in Shenyang Agricultural University in 2022 and 2023, using a medium-late maturing japonica rice cultivar 'Beijing 1501' as the test material. Four treatments were established: no straw return (CK), straw return in spring (ST), straw return in autumn with wet harrowing (SW), and rice straw biochar return in spring (SC). Rice grain yield, primary and secondary panicle branch traits, dry matter weight of each organ, net photosynthetic rate, leaf area index, N accumulation in different organs, and N use efficiency at the key growth stages were measured, and the relationships between grain yield and dry matter accumulation as well as N uptake and utilization were analyzed.

Results The SC and SW treatments, while maintaining a certain panicle number, increased spikelets per panicle and improved secondary panicle branch traits; yield under SC and SW were both significantly higher than under ST, whereas the yield under ST was comparable to that under the CK. In both years, the SC and SW treatments showed higher net photosynthetic rates at the full heading and filling stages than the ST treatment, whereas the ST treatment showed lower values than CK. The SW and SC treatments increased total dry matter and total N accumulation at maturity, whereas the ST treatment was lower than CK. Furthermore, the SC and SW treatments enhanced N recovery efficiency, agronomic efficiency, partial factor productivity, and soil N dependence rate, with most of these indices attaining their highest values under SC. Both correlation analysis and principal component analysis (PCA) indicated that total dry matter accumulation at maturity, N recovery efficiency, and N partial factor productivity were highly significantly and positively correlated with yield.

Conclusions Both straw return in autumn with wet harrowing and applying rice straw as biochar enhanced rice photosynthetic efficiency, dry matter, and N accumulation at maturity, thereby promoting rice yield and N use efficiency. By contrast, straw return in spring reduced the rice N uptake capacity and increased the risk of yield loss. Therefore, return straw in autumn coupled with wet harrowing, or applying straw as biochar in spring are suitable methods of achieving high yields of japonica rice in northern cold regions and for the efficient utilization of straw.